1. Field of the Invention
The present invention relates to solid electrolytic substances, particularly, to solid electrolytic substances having a good conductivity of oxygen ions. Such substances are advantageously used in an oxygen sensor which takes advantage of the potential differences produced in response to the movement of oxygen ions when they contact a substance having different oxygen partial pressures at its opposite sides such substances are also advantageously used in an oxygen pump, fuel cell etc. which take advantage of the movement of oxygen ions in response to the application of electric current thereto.
2. Description of the Prior Art
In an oxygen sensor used to measure the oxygen partial pressure in the high temperature range of, for example, 500.degree..about.1500.degree. C., solid electrolytic substances made of ZrO.sub.2 having Y.sub.2 O.sub.3 or CaO and MgO added thereto have been used. Such sensors are used in connection with two measuring methods.
One measuring method is called a sampling method, wherein a gas to be measured is introduced into a measuring apparatus through an induction pipe and is reheated to a temperature of .about.1000.degree. C. to allow sufficient reactions of the solid electrolytic substance therein.
The other measuring method is called the direct insertion method. In this method the solid electrolytic substances are used as a barrier between a gas to be measured and a standard gas, and are inserted directly into the gas to be measured.
In either method, the potential difference E of the standard gas and the gas to be measured is read by a potentiometer. The oxygen partial pressure of the gas to be measured is then calculated in accordance with the following Nernst equation: ##EQU1## where,
R=gas constant, T=absolute temperature,
F=Faraday constant, Po.sub.2 (R)=oxygen partial pressure of standard gas and Po.sub.2 (S)=oxygen partial pressure of gas to be measured.
In the sampling method, since gas to be measured is reheated to a constant temperature in the measuring apparatus, the oxygen partial pressure can be measured simply. However, while there is no problem if the oxygen partial pressure is not changed by such a temperature as adding O.sub.2 to N.sub.2, it is ineffective when the oxygen partial pressure is changed by such a temperature as adding H.sub.2 O and H.sub.2 to N.sub.2.
With the oxygen sensor employed in the direct insertion method, the aforesaid problem is not encountered since the measurement is made directly. However, the more severe the measurement conditions, the shorter the life time of the sensor.
FIG. 2 is a sectional explanatory view showing an example of an oxygen sensor of a conventional direct insertion method which is a background of the present invention. The oxygen sensor 1 includes a pipe 2 sealed at one end and consists of solid electrolytic substances of ZrO.sub.2 to which Y.sub.2 O.sub.3 or CaO and MgO have been added. The pipe 2 has been provided with porous platinum electrodes 3a and 3b, baked onto its inner and outer surfaces respectively. In the oxygen sensor 1, a standard gas is introduced into the pipe 2, which is inserted into the gas to be measured to measure the oxygen partial pressure.
In the direct insertion method shown in FIG. 2, since the pipe 2 is inserted into the gas to be measured, it must be relatively long and is expensive to provide. Besides, in the conventional oxygen sensor 1 used in the insertion method shown in FIG. 2, the pipe 2 consisting of a solid electrolytic substance is susceptible to thermal shocks and its heat-resisting cycle is short with the result that its frequency of failure is very high.
An oxygen sensor has also been devised in which an oxygen sensor chip comprising a columnar element consisting of solid electrolytic substances of ZrO.sub.2 added with Y.sub.2 O.sub.3 or CaO and MgO, and formed with porous platinum electrodes on its opposite end faces, is adhered or fused to the end portion of a non-electrolytic ceramic pipe. In such an oxygen sensor a non-electrolytic ceramic pipe, for example, an alumina pipe, mullite pipe etc. was used which is very hard at high temperatures. A pipe consisting of solid electrolytic substances was not used so that the pipe could, be manufactured at low cost and would result in a low frequency of failure. However, a problem arose due to the difference in thermal expansion coefficients between the ceramic pipe (non-electrolyte) and the oxygen sensor chip (solid electrolytic substance). The difference in thermal expansion coefficients caused cracks to occur after repeated use which could deteriorate the airtightness and disturb the accuracy of the measurement of the oxygen partial pressure.